Previous electrically programmable ROM's have been programmed by causing a permanent and irreversible change in the memory metal interconnection pattern by electrical pulses. Some ROM's have been programmed by causing a reversible change in the characteristics of a memory device within the ROM. The so-called alterable ROM's were generally deprogrammed by exposing the unpackaged device to ultraviolet light or by exposing a sealed package containing the ROM to a dose of X-ray radiation.
One such alterable ROM is the FAMOS Read-Only-Memory such as that described in the IEEE Journal of Solid-State Circuits, Volume SC-6, No. 5, October, 1971, by Dov Frohman-Bentchkowsky. Commercial devices using this technology are the 1602A, 1702A and 1302 alterable ROM's manufactured by Intel. These devices contain a quartz lid which allows erasure, i.e., the returning of the internal devices to the unprogrammed state, through the use of high-intensity, ultraviolet light.
In accordance with the preferred embodiment of the present invention, an alterable ROM is provided which may be field-programmed by electrical pulses. The information stored therein is subsequently held for an indefinite period. However, in contrast to previous alterable ROM's, the memory cell of the preferred embodiment may be deprogrammed by subsequent electrical pulses. A P-channel mosfet and an N-channel mosfet are constructed on a single chip using a common floating gate. By using a common floating gate for the P and N-channel devices, it is possible to cause avalanche injection of either holes or electrons into the floating gate, thereby representing one of two distinct states. Furthermore, by subsequently reversing the programming voltages applied, it is possible to reprogram the memory cell and change the stored value. No additional power is required to maintain the memory cell between reprogramming. Furthermore, the cell can be interrogated through nondestructive methods which use voltages less than that required for programming.